Liquid crystal device with plural ferroelectric or antiferroelectric layer tilt angles per pixel

ABSTRACT

A liquid crystal device used for electro-optical devices, in which each parameter of the liquid crystal having layer structures is specified to provide a liquid crystal high in stress resistance and capable of gray scale display. The liquid crystal held between a pair of parallel substrates is either ferroelectric or antiferroelectric and represents layer structures. A plurality of substrate interface tilt angles can be set for the layer structures. The layer structures can be configured asymmetric or symmetric about a plane intermediate between the parallel substrates. With this configuration, a liquid crystal device is constructed having a plurality of layer structures including at least one layer structure, which has (1) a substrate interface layer tilt angle of 0° to 3° and the c director pretilt of 3° to 5°, or (2) a substrate interface layer tilt angle of 4° to 7° and the c director pretilt of 0° to 3°, or (3) a substrate interface layer tilt angle of 8° to 20 ° and the c director pretilt of 9° to 90° or less.

TECHNICAL FIELD

The present invention relates to a liquid crystal device, and inparticular to a liquid crystal device used for electro-optic devicesusing a liquid crystal display element or a liquid crystal opticalshutter array. The invention relates, in particular, to a liquid crystaldevice using a ferroelectric liquid crystal or an antiferroelectricliquid crystal characterized by layer structures and moleculararrangement.

BACKGROUND ART

A liquid crystal device using a ferroelectric liquid crystal, asdisclosed by Clark, et al., utilizes the spontaneous polarization ofliquid crystal molecules. The spontaneous polarization of theferroelectric liquid crystal is attributable to the molecular structure.Specifically, in a structure of a racemic modification having asymmetriccarbon in molecules with a dipole moment perpendicular to the major axisof the molecule, the dipole moments are aligned in the same direction tothereby develop a spontaneous polarization in a SmC* phase with theliquid crystal molecules aligned along the major molecular axis and therotation thereof around the major axis is hindered. In the ferroelectricliquid crystal, two stable molecular arrangements can be secured bycontrolling the direction of spontaneous polarization by applying anelectric field from an external source. Of these two states, onedisplays white and the other displays black. These stable states areheld after the electric field is removed, and therefore have a memorycharacteristic. Also, the response of the ferroelectric liquid crystalto the voltage application, which is derived from the primary couplingwith the electric field, is so high that the ferroelectric liquidcrystal is expected to replace the paraelectric liquid crystal. In viewof this, many attempts have so far been made to use the ferroelectricliquid crystal in practical applications.

However, the ferroelectric liquid crystal is a system having asymmetriccarbon and therefore has a spiral characteristic. Also, since layerstructures are formed in the SmC* phase, it is difficult to control thedirection of spontaneous polarization. As a result, a liquid crystaldevice has been difficult to produce using the ferroelectric liquidcrystal having a bistable characteristic and a memory characteristic aspresented by Clark, et al. Especially, the layer structures of theferroelectric liquid crystal are so complex that a twist structure, achevron structure, etc. are known in addition to the book shelfstructure initially suggested. It has thus become apparent that thelayer structures are involved in the various characteristics of theliquid crystal device using the ferroelectric liquid crystal.

As part of study on the molecular orientations including the layerstructures, Kanbe et al., disclosed a liquid crystal device (UnexaminedPatent Publication (Kokai) No. 63-124030) using a ferroelectric liquidcrystal in which a c director has a pretilt rotationally symmetric aboutthe substrate center in the neighborhood of each of the two substrates,exhibiting the chevron layer structures and a spray orientation("Next-Generation Liquid Crystal Display and Liquid Crystal Material",published by CMC, 1992).

The chevron layer structures, however, have a stable moleculararrangement, at the time of driving, different from the stable moleculararrangement at the time of storage, and therefore has the disadvantagethat the contrast is low at the time of storage and flickering occurs atthe time of driving. Further, with the ferroelectric liquid crystal,which is inherently an enantiomeric system having asymmetric carbon, thechevron layer structures cannot be considered bistable in terms ofenergy. Therefore, the stability of the ferroelectric liquid crystal,after being kept in stock in the stable state for long time, increasesto such an extent that it is no longer possible to switch to the otherstate (this phenomenon is hereinafter called "unilaterally stable"),thereby leading to a drawback in the lack of long-term reliability.

Further, these layer structures are not as strong as crystal and easilysuccumb to stress and break under any small external pressure which maybe exerted on the substrate. Once the layer structures of the liquidcrystal device using the ferroelectric liquid crystal are broken, itmust be heated up to the isotropic temperature and then cooled torebuild the layer structures.

Similar layer structures in the antiferroelectric liquid crystal, on theother hand, have a higher rigidity than the layer structures of theferroelectric liquid crystal. This is considered primarily due to themulti-domain structure. The multi-domain structure is a state in whichfine layer structures coexist in a minute area at wavelength level.Conceptually, it is a state in which many pillars stand in a small area.The high rigidity of the layer structures of the antiferroelectricliquid crystal is considered to be derived from the antagonism betweenthese layers.

In the antiferroelectric liquid crystal, the field induced phasetransition to the ferroelectric phase occurs under an electric field,and depending on the polarity of the electric field exerted, two stablemolecular arrangements are obtained as in the case of the ferroelectricliquid crystal. The antiferroelectric liquid crystal, however, isdifferent from the ferroelectric liquid crystal in that it has no memorycharacteristic with the two stable molecular arrangements. Once theelectric field is removed, therefore, the antiferroelectric liquidcrystal alternates between these two stable molecular arrangements inevery other layer and retransition occurs to the antiferroelectric phasewhere the spontaneous polarization is cancelled. Further, theantiferroelectric liquid crystal has a plurality of subphases (ferriphases) called the demon's steps. The possibility of gray scale displayis being studied using the field induced phase transition to this ferriphase. Since the field induced phase transition sensitively responds toa slight voltage difference, however, selective partial phase transitionis difficult, and therefore a satisfactory gray scale display has notyet been obtained.

In the ferroelectric liquid crystal, since the ferroelectric liquidcrystal has no subphase unlike the antiferroelectric liquid crystal,gray scale display by partial switching is impossible to realize. Also,the ferroelectric liquid crystal, which has a monodomain instead of amultidomain structure, has a uniform layer structure over a wide area.Therefore, the ferroelectric liquid crystal is constructed the same wayas if a large roof is supported by a small number of pillars, and easilysuccumbs to stress.

DISCLOSURE OF THE INVENTION

Accordingly, the object of the present invention is to provide a liquidcrystal device with a structure that can obviate the disadvantagesattributable to the conventional layer structures of a liquid crystaldevice using a liquid crystal having the layer structures.

Specifically, the object of the invention is to provide a reliableliquid crystal device using the ferroelectric liquid crystal, in whichgray scale display is realizable with high contrast without any contrastdifference between storage time and drive time, no flicker occurs at thetime of drive, and a long-term bistability is assured on the one hand,and the liquid crystal device using the antiferroelectric liquid crystalis capable of selectively controlling the field induced phase transitionon the other hand.

In order to achieve the above-mentioned object, according to a firstaspect of the invention, there is provided a liquid crystal devicecomprising a pair of parallel substrates each having an electrode and aliquid crystal with layer structures held between the substrates so asto form pixels between the electrodes, characterized in that the layerstructures are arranged in such a manner that there are at least twosubstrate interface layer tilt angles formed between the normal to oneof the substrates and the layer structure plane in the same pixel, atleast one layer in the same pixel has the substrate interface tilt angleof selected one of (1) 0° to 3°, (2) 4° to 7°, and (3) 8° to 20°, andthe c director pretilt providing an angle between the c directorconstituting a unit vector of projection of the liquid crystal moleculeson the layer plane at the substrate interface and the component of thelayer plane parallel with the substrate is selected one of 3° to 5° for(1), 0° to 3° for (2), and 9° to 90° for (3) above.

In the process, the layer structures in the same pixel are partly orwholly formed asymmetric or symmetric about a symmetry plane equidistantfrom the two substrates, and a liquid crystal is ferroelectric orantiferroelectric.

In order to achieve the above-mentioned objects, according to a secondaspect of the invention, there is provided a liquid crystal devicecomprising a pair of parallel substrates each having an electrode and aferroelectric liquid crystal with layer structures held between thesubstrates so as to form pixels between the electrodes, characterized inthat the c director providing a unit vector of projection of the liquidcrystal molecules on the layer structure plane at the substrateinterface in the same pixel is arranged symmetrically about a symmetryplane equidistant from the two substrates, the substrate interface layertilt angle between the layer plane and the normal to one of thesubstrates is selected one of (1) 0° to 3°, (2) 4° to 7°, and (3) 8° to20°, and the c director pretilt providing an angle between the cdirector and the component of the layer plane parallel with thesubstrates is 3° to 5° for (1), 0° to 3° for (2) and 9° to 90° for (3)above.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages will be madeapparent by the detailed description of embodiments taken in conjunctionwith the accompanying drawings, in which:

FIG. 1A is a perspective view showing a first model of layer structuresand a molecular arrangement in a liquid crystal device according to thepresent invention;

FIG. 1B is a perspective view showing the range in which a single liquidcrystal molecule is movable;

FIG. 1C is a bottom view of the cone of FIG. 1B taken from theperpendicular direction to the layer plane.

FIG. 1D is a sectional view showing the layer structures of the firstmodel of FIG. 1A taken in the direction of arrow A;

FIG. 1E is a sectional view showing the structure of anantiferroelectric liquid crystal layer adjacent to the layer of FIG. 1Dtaken in the direction of arrow A in FIG. 1A;

FIG. 1F is a sectional view taken in the plane perpendicular to thesubstrates of the first model shown in FIG. 1A;

FIG. 1G is a diagram for explaining various forms of layer structureshaving at least two angles in an actual liquid crystal device;

FIG. 1H is a diagram for explaining the pixels of a liquid crystaldevice by reference to the scanning electrodes and the signalelectrodes;

FIG. 2A is a model diagram showing a second model of the layerstructures and molecular arrangement in a liquid crystal deviceaccording to the invention using a ferroelectric liquid crystal, inwhich the liquid crystal device with the c director thereof viewed fromthe layer plane is partially enlarged;

FIG. 2B is a sectional view taken in line B--B in FIG. 2A;

FIGS. 3A to 3D are diagrams showing a first example configuration of thelayer structures and the molecular arrangement of a liquid crystaldevice according to the invention, in which the distribution along yaxis of the y-axis component of the a director and the x-axis, y-axisand z-axis components of the c director are shown along the abscissawith the distance between the substrates specified as unity;

FIG. 3E is a sectional view of the layer structures of a liquid crystaldevice according to the invention as estimated from FIG. 3A;

FIGS. 4A to 4D are diagrams showing a second example configuration ofthe layer structures and the molecular arrangement of a liquid crystaldevice according to the invention, in which the distribution along the yaxis of the y-axis component of the a director and the x-axis, y-axisand z-axis components of the c director are shown along the abscissawith the distance between the substrates specified as unity;

FIG. 4E is a sectional view of the layer structures of a liquid crystaldevice according to the invention as estimated from FIG. 4A;

FIGS. 5A to 5D are diagrams showing a third example configuration of thelayer structures and the molecular arrangement of a liquid crystaldevice according to the invention, in which the distribution along the yaxis of the y-axis component of the a director and the x-axis, y-axisand z-axis components of the c director are shown along the abscissawith the distance between the substrates specified as unity;

FIG. 5E is a sectional view of the layer structures of a liquid crystaldevice according to the invention as estimated from FIG. 5A;

FIGS. 6A to 6D are diagrams showing a fourth example configuration ofthe layer structures and the molecular arrangement of a liquid crystaldevice according to the invention, in which the distribution along yaxis of the y-axis component of the a director and the x-axis, y-axisand z-axis components of the c director are shown along the abscissawith the distance between the substrates specified as unity;

FIG. 6E is a sectional view of the layer structures of a liquid crystaldevice according to the invention as estimated from FIG. 6A;

FIGS. 7A to 7D are diagrams showing a fourth example configuration ofthe layer structures and the molecular arrangement of a liquid crystaldevice not included in the invention, in which the distribution along yaxis of the y-axis component of the a director and the x-axis, y-axisand z-axis components of the c director is shown along the abscissa withthe distance between the substrates specified as unity;

FIG. 7E is a sectional view of the layer structures of a liquid crystaldevice according to the invention as estimated from FIG. 7A;

FIG. 8 is a diagram showing the relation between a polarization vector pand other vector parameters of a ferroelectric liquid crystal;

FIG. 9A is a diagram showing an evaluation test waveform (white writewaveform) according to an embodiment of the invention;

FIG. 9B is a diagram showing the transmittance after application of anevaluation test waveform (white write waveform) according to anembodiment of the invention;

FIG. 10A is a diagram showing an evaluation test waveform (black writewaveform) according to an embodiment of the invention;

FIG. 10B is a diagram showing the transmittance after application of anevaluation test waveform (black write waveform) according to anembodiment of the invention; and

FIG. 11 is a graph showing the result of the unilaterally stableevaluation according to an embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1A to 1F are model diagrams showing a molecular arrangement (cdirector) and layer structures of a ferroelectric liquid crystal and anantiferroelectric liquid crystal. As shown in FIG. 1A, in a liquidcrystal device according to the present invention, liquid crystalmolecules 1 are held between a pair of parallel substrates 21, 22, andlayer structures 3 are formed as the centers of gravity of the liquidcrystal molecules 1 are aligned from the lower substrate to the uppersubstrate. Also, reference numeral 23 in FIG. 1A designates a symmetryplane equidistant from the substrates 21, 22. In the first model diagramshown in FIG. 1A, part or the whole of the layer structures 3 in thesame pixel is asymmetric about the symmetry plane 23 equidistant fromthe substrates 21, 22.

Explanation will be made hereinafter in this specification on theassumption that the x axis and z axis exist in the plane parallel withthe substrates 21, 22 and the y axis exists in the directionperpendicular to the substrates 21, 22 as shown in FIG. 1A.

The range within which a single liquid crystal molecule 1 is movable ispresented as a conical model shown in FIG. 1B. The bottom of the cone islocated at the same position as the layer plane 31. FIG. 1C is a bottomview of the cone of FIG. 1B taken from the direction perpendicular tothe layer plane 31. As shown in FIGS. 1B, 1C, the unit vector ofprojection of the liquid crystal molecule 1 on the layer plane (conebottom) 31 is called the c director 4. Also, the angle 6 that the cdirector 4 forms with the component of the layer plane 31 parallel withthe substrates 21, 22 is called a c director pretilt 6 herein.

FIG. 1D is a view of the layer structures 3 of the first model of FIG.1A taken from the direction of arrow A (along the z-axis). In thechevron-type layer structures, the layer plane 31 is inclined and,therefore, the bottom of the cone existing in the layer plane 31 of FIG.1D appears elliptical. Although only one layer is plotted in FIG. 1D,layer structures similar to the layer structure 3 shown in FIG. 1D areformed continuously in the ferroelectric liquid crystal. Theferroelectric liquid crystal is so structured that all the c directors 4are positioned in the same direction.

An antiferroelectric liquid crystal also has similar layer structures 3.In the layer structures 3 of the antiferroelectric liquid crystal,however, the c directors 4 are aligned alternately in differentdirections in the odd-numbered layers and the even-numbered layers.Consequently, assuming that the layer structure 3 shown in FIG. 1D is anodd-numbered layer structure of the antiferroelectric liquid crystal,the even-numbered layer structure 3 of the antiferroelectric liquidcrystal takes the form as shown in FIG. 1E.

FIG. 1F is a sectional view of the first model of FIG. 1A taken in aplane perpendicular to the x axis. In FIG. 1F, reference numeral 5designates the angle that the layer plane 31 forms with a normal 20 toone of the substrates 21, 22. The angle 5 is hereinafter called thesubstrate interface layer tilt angle 5.

Now, a second model of the present invention will be explained withreference to FIGS. 2A, 2B. In the second model, the same component partsas the corresponding ones in the first model are designated by the samereference numerals, respectively.

FIG. 2A is a model diagram taken from the direction along the z-axisshowing the layer plane 31 containing the c director 4 providing a unitvector of projection of the liquid crystal molecules 1 of theferroelectric liquid crystal on the layer plane 31, with a part of theliquid crystal device shown enlarged. FIG. 2B shows a sectional viewtaken in line B--B in FIG. 2A.

As shown in FIGS. 2A, 2B, the second model of the liquid crystal devicecomprises liquid crystal molecules 1 of the ferroelectric liquid crystalheld between a pair of parallel substrates 21, 22, and each of theliquid crystal molecules 1 has the center of gravity thereof alignedfrom the lower substrate to the upper substrate thereby to form a layerstructure 3. Also, numeral 23 designates a symmetry plane equidistantfrom the substrates 21, 22. In the second model shown in FIGS. 2A, 2B,the layer structures 3 in the same pixel are partly or wholly symmetricabout the symmetry plane 23 equidistant from the substrates 21, 22.

Now, the present invention will be explained in detail on the basis ofthe first and second models described above using parametersrepresenting the structural arrangement of the layer structures 3including the a director 7 providing a layer normal vector locatedperpendicular from the layer plane 31 and the c director 4 providing aparameter representing a molecular arrangement.

In the above-mentioned model diagram, the tilt of the layer plane 31 isthe same for all the layer structures 3. An actual liquid crystaldevice, which has at least two types of the substrate interface layertilt angle 5 that the normal 20 to one of the two substrates forms withthe layer plane 31 of the layer structure 3 in a single pixel, as shownin FIG. 1G, takes various forms of layer structures. The pixel hereinreferred to is, as shown in FIG. 1H, a portion (indicated by hatching)Anm at a crossing between a given scanning electrode Xn (n=1 to 480) anda given signal electrode Ym (m=1 to 640) formed on the two substrates.

An explanation will be given about the ATR (attenuated total reflection)method providing means for checking the a director 7 and the c director4 and a method of determining the layer structures and the moleculararrangement in an actual liquid crystal cell.

The ATR method, which is described in detail in the thesis by Sambles,et al., entitled "Liquid Crystals", 1993, Vol. 13, No. 1, 1-11, etc., isbriefly explained below.

Suppose that a liquid crystal cell is prepared with a film exhibiting anabsorption characteristic (normally, a vapor-deposited gold or a silverfilm 30 to 50 nm thick) as an electrode and a p-polarization light(having an electric field vector in the plane of incidence) is appliedto this liquid crystal cell. The light is reflected substantially in itsentirety by the metal film up to a predetermined angle of incidence.When the total reflection angle specified by the dielectric tensor isexceeded, however, a phenomenon is known to occur in which light energyis exchanged with the metal film, thereby exhibiting a unique reflectionintensity profile determined by the metal film, the orientation film andthe liquid crystal.

This reflection intensity profile is sensitive especially to theproperties (the layer structures and the molecular arrangement of theliquid crystal) of the material directly under the metal film. The layerstructures and the molecular arrangement of the liquid crystal cantherefore be fully and specifically examined from the relation betweenthe absolute value of reflection intensity and the angle of incidenceassociated with the particular intensity (hereinafter referred to as"the ATR curve").

Now, a method of determining the layer structures and the moleculararrangement will be described. The c director and the a director areused for describing layer structures and a molecular arrangement. Thefree energy density F of a liquid crystal taking the elastic deformationof the c director and the a director into consideration is given by thefollowing equation ("Liquid Crystals", by Nakagawa, 1990, Vol 8, No. 5,651-675).

    F=A/2*(∇·a).sup.2 +{(∇·c).sup.2 +(∇×c).sup.2 }-D*c·∇×c +D1*ν·∇×c -C*(∇·a)*(∇·c)+L/2*(κ-κ0).sup.2                                                      (1)

where capital letters A, C, D, D1 and L designate elastic constantsrepresenting the deformation of the layer and the molecular arrangement.Also, the relation described below holds between the parameters.

    κ=a·ν-1                                  (2)

    κ0=dA/dc*                                            (3)

    p=ν×c                                             (4)

    a ∥ν                                           (5)

where dA is the layer thickness at smectic A phase, dc* the layerthickness at smectic C* phase, ν the unit vector in the direction of thenormal to the layer, and p the polarization vector having an orthogonalrelation as shown in FIG. 8 with the a and c vectors.

An actual liquid crystal cell assumes layer structures and a moleculararrangement stable in terms of energy when the value of the energydensity given by equation (1) above integrated over the entire bulk isin equilibrium with the surface anchoring energy Fs (Fs0 for the lowersubstrate, and Fsd for the upper substrate) indicating the restrictionfrom the interface, as follows.

    Fs0=-g/2 exp{-σ*sin((Φ-Φ0)/2)}+exp{-σ*cos((Φ+Φ0)/2)}!                                                     (6)

    Fsd=-g/2 exp{-σ*sin((Φ-Φd)/2)}+exp{-σ*cos((Φ+Φd)/2)}!                                                     (7)

where g and σ are coefficients, Φ0 is the c director pretilt 6 for thelower substrate side, and Φd is the c director pretilt 6 for the uppersubstrate side.

The elastic constant, the coefficients and the parameters are fitted insuch a manner as to coincide with the ATR curve determined by numericalcalculations based on equations (1), (6) and (7) and experimentallyobtained for the particular layer structures and the moleculararrangement. In this way, layer structures and a molecular arrangementstable, in terms of energy, can be confirmed in the liquid crystal cell.

The energy level is determined by the calculations based on equations(1), (6) and (7) using the c director and the a director of the layerstructures and the molecular arrangement obtained by the fitting of theATR curve.

An example method of fabricating an actual liquid crystal cell will beexplained as an embodiment of the invention.

FIRST EMBODIMENT

According to the first embodiment, an ITO electrode is formed on each ofa pair of glass substrates with an orientation film coated on each ofthe electrodes. Different orientation film materials have differentoptimum values of thickness and other conditions for forming anorientation film. The first embodiment will be described with referenceto the case in which Hoechst's Polix008 is used as an orientation film.Two types of chiral materials having different pitches are mixed at 0.5%each with Polix008, and a film is formed to the thickness of about 6 nmon the ITO electrode on the glass substrate. After that, the film isrubbed.

The rubbing process is performed in a direction substantially coincidentwith the z axis shown in FIGS. 1A, 1D, 1E and 1F. A pair of substratesare arranged with the orientation film surfaces thereof in opposedrelation to each other, and baked under pressure with the peripheralportions thereof fixed by resin. The ferroelectric liquid crystalmaterial Felix-T252 made by Hoechst is injected, the injection hole issealed by resin, and the isotropic processing is performed, thuscompleting a liquid crystal cell. The phase series of Felix-T252 isshown below.

X→(-8)→SmC*→(54)→SmA→(76)→N*.fwdarw.(80)→I

where X represents crystal, SmC* the smectic C* phase, SmA the smectic Aphase, N* the cholesteric phase, and I the isotropic phase. The figuresin the parentheses indicate the temperature (° C.) for transition toeach phase.

The liquid crystal cell thus obtained is subjected to AC fieldprocessing by applying a rectangular wave of 30 volts, 30 Hz thereto forone minute. The measurement conducted on this liquid crystal cell by theATR process confirmed the characteristic indicating the layer structuresand the molecular arrangement as shown in FIGS. 3A to 3D and thecharacteristic indicating the layer structures and the moleculararrangement as shown in FIGS. 5A to 5D in the same pixel.

Explanation will be made about the liquid cell configuration obtainedfrom the characteristics representing the layer structures and themolecular arrangement shown in FIGS. 3A to 3D and FIGS. 5A to 5D.

FIGS. 3A to 3D and FIGS. 5A to 5D show the distribution along the y axisof the y-axis component ay of the a director, and the x-axis componentcx, the y-axis component cy and the z-axis component cz of the cdirector, with the distance between the substrates along the abscissaspecified as unity.

The substrate interface layer tilt angle 5 is estimated from the valuesalong the ordinate at 0.0 and 1.0 on the abscissa of the y-axiscomponent ay of the a director (the value along the ordinate at 0.0 onthe abscissa represents the value for the upper substrate, and the valuealong the ordinate at 1.0 on the abscissa gives the value for the lowersubstrate). Also, the geometry of the layer structures is estimated fromthe traces of the y-axis component ay of the a director. Morespecifically, the angle of the c director in the cell can be calculatedanywhere from Tan⁻¹ (ay).

The substrate interface layer tilt angle 5 of 4° was obtained from FIG.3A. The layer structures with the substrate interface tilt angle 5 of 4°can be estimated as shown in FIG. 3E.

Also, the c director pretilt 6 at the substrate interface can becalculated from the values along the ordinate at 0.0 and 1.0 on theabscissa of the x-axis component of the c director shown in FIG. 3B. Thedirector pretilt 6 of 1° could be obtained from FIG. 3B. The sign of thex-axis component cx of the c director is positive and constant in FIG.3B. Even in the case where the x-axis component cx of the c director isnegative and constant, however, the energy calculated from equation (1)was equivalent.

In a similar fashion, the substrate interface tilt angle 5 obtained fromFIG. 5B was 10°, and the c director pretilt 6 at the substrate interfacewas 10°. In this configuration, the y-axis component ay of the adirector continuously changes while being asymmetrically distributedfrom 0.0 to 1.0 about a symmetry axis at equal distance from thesubstrate, i.e., at 0.5 on the abscissa. This indicates that the layerstructures are asymmetric about a symmetry plane equidistant from thetwo substrates. The layer structures with the substrate interface layertilt angle 5 of 10° is estimated as shown in FIG. 5E.

In FIG. 5B, the x-axis component cx of the c director is negative andconstant. As in the case of FIG. 3B, a positive and constant x-axiscomponent cx of the c director is equivalent in terms of energy.

Also, the characteristic exhibited by the layer structures and themolecular arrangement as shown in FIGS. 4A to 4D, the characteristicexhibited by the layer structures and the molecular arrangement as shownin FIGS. 6A to 6D and the state in which these layer structures and themolecular arrangements coexist in the same pixel could be confirmed byregulating the density, types, numbers, properties, etc. of the chiralmaterial mixed with the orientation film of the liquid crystal cellprepared according to the first embodiment.

FIGS. 4A to 4D and FIGS. 6A to 6D, like FIGS. 3A to 3D and FIGS. 5A to5D, represent an example of the characteristic exhibiting theconfiguration of the layer structures and the molecular arrangement.FIGS. 4A to 4D and FIGS. 6A to 6D show a distribution along the y axisof the y-axis component ay of the a director, and the x-axis componentcx, the y-axis component cy and the z-axis component cz of the cdirector with the distance between the two substrates specified asunity.

From the characteristics of FIGS. 4A and 4B, the substrate interfacelayer tilt angle 5 can be calculated as 1°, and the c director pretilt 6at the substrate interface as 3°. The layer structure with the substrateinterface layer tilt angle 5 of 3° is estimated as shown in FIG. 4E.Also, the x-axis component cx of the c director in FIG. 4B is positiveand constant. A negative x-axis component cx of the c director is alsoequivalent in terms of energy as in the case of FIG. 3B.

From the characteristics of FIGS. 6A and 6B, the substrate interfacelayer tilt angle 5 can be calculated as 15°, and the angle of the cdirector pretilt 6 at the substrate interface can be calculated as 15°from the substrate plane. In the configuration of the liquid crystalcell obtained from these characteristics, though not so conspicuous asin the case of FIGS. 5A, 5B, the y-axis component ay of the a directorcontinuously changes while being asymmetrically distributed from 0.1 to1.0 on the abscissa assuming that a symmetry axis is taken at 0.5 on theabscissa between a pair of substrates. The layer structure having thissubstrate interface layer tilt angle 5 of 15° is estimated as shown inFIG. 6E. Also, the x-axis component cx of the c director is negative andconstant. In this case, as in the case of FIGS. 3A and 3B, the x-axiscomponent cx of the c director, if positive, is equivalent in terms ofenergy.

SECOND EMBODIMENT

A liquid crystal cell was prepared according to the second embodimentunder the same conditions as in the first embodiment, except thatPolix008of Hoechst in the first embodiment was not mixed with the chiralmaterial but used as an orientation film in the present case.

The liquid crystal cell obtained in this manner was impressed with arectangular wave of 30 volts, 30 Hz for one minute and thus subjected toAC field processing. This liquid crystal cell was measured by the ATRmethod, with the result that the characteristics exhibiting the layerstructures and the molecular arrangement could be confirmed as shown inFIGS. 3A to 3D.

From the value of the x-axis component cx of the c director along theordinate at 0.0 on the abscissa shown in FIG. 3B, the c director pretilt6 for one of the substrate interfaces can be calculated. Also, it ispossible to calculate the c director pretilt 6 for the other substrateinterface from the value of the x-axis component cx of the c directoralong the ordinate at 1.0 on the abscissa. In FIG. 3B, the values alongthe ordinate are equal at points 0.0 and 1.0 on the abscissa, andtherefore the c director pretilts 6 at the two substrate interfacescould be calculated as 10°. Also, the x-axis component cx of the cdirector is positive and constant in FIG. 3B. The c director at thesubstrate interface, therefore, was found to be arranged in planesymmetry.

The first embodiment represents at least two types of layer structures.The second embodiment, on the other hand, is the one in which the cdirector is symmetric, and has a plurality of layers in the same pixelas described above. Consequently, it follows that the second embodimenthas the same layer structures as the first embodiment, and the layerstructures estimated from FIGS. 3A to 3D are as shown in FIG. 3E.

THIRD EMBODIMENT

Next, a liquid crystal cell was prepared according to the thirdembodiment by changing the material of the orientation film used for oneof the substrates in the second embodiment while using the same materialfor the orientation film of the other substrate as in the firstembodiment. The orientation films were prepared by forming Polix008 ofHoechst on the electrode of one of the substrates, and Polix004 ofHoechst on the electrode of the other substrate. The films thus formedwere rubbed, assembled in the same procedure as in the secondembodiment, and subjected to a similar test as in the second embodiment.As a result, the characteristics showing a configuration of the layerstructures and the molecular arrangement as shown in FIGS. 4A to 4Dcould be confirmed from the ATR curve.

According to the characteristics shown in FIGS. 4A to 4D, the c directorpretilts 6 at the two substrate interfaces were calculated as 3°,respectively. The x-axis component cx of the c director is positive andconstant. Therefore, it was found that the c director at the substrateinterface is arranged in plane symmetry. Layer structures as shown inFIG. 4E can be estimated from FIGS. 4A to 4D.

FOURTH EMBODIMENT

A liquid crystal cell according to the fourth embodiment was prepared bychanging the orientation film used for one of the substrates in thesecond embodiment and using the same material as in the second and thirdembodiments for the other substrate. Polix008 of Hoechst was formed andrubbed on the electrode of one of the electrodes, and an LB film wasformed and assembled in the same procedure as in the preceding cases onthe electrode of the other substrate. Then, a similar experiment wasconducted. From the ATR curve, a characteristic representing theconfiguration of the layer structures and the molecular arrangement asshown in FIGS. 5A to 5D could be confirmed.

In FIGS. 5A and 5B, the c director pretilt 6 at the two substrateinterfaces was 10°. The x-axis component cx of the c director isnegative and constant, and therefore the c director at the substrateinterface was found to be arranged in plane symmetry. The layerstructures as shown in FIG. 5E are estimated from FIGS. 5A to 5D.

REFERENCE

A liquid crystal cell was prepared in a manner similar to the firstembodiment and subjected to AC field processing with a voltage valuelower than in the first embodiment, thus preparing a reference. Theliquid crystal cell according to this reference was measured by the ATRmethod. As a result, the characteristics representing all the layerstructures and molecular arrangements in the same pixel were found asshown in FIGS. 7A to 7D.

FIGS. 7A to 7D show example characteristics of the configuration of thelayer structures and the molecular arrangement not suitable for thepresent invention. FIGS. 7A, 7B were found to relate to the case inwhich the substrate interface layer tilt angle 5 is 4° and the cdirector pretilt at one of the substrate interfaces is 10°. The layerstructures with the substrate interface layer tilt angle 5 of 4° isestimated as shown in FIG. 7E.

In this configuration, the x-axis component cx of the c director changesfrom positive to negative between a pair of substrates, in what iscalled the spray orientation. In this case, the value of the c directorpretilt 6 is different between one substrate interface and the othersubstrate interface. These pretilts 6 lack plane symmetry. As describedabove, the spray orientation appears in other than the configurationshown in the invention, thereby making it impossible to secure asufficient contrast.

For the layer structures not to be formed with the spray orientation asshown in FIGS. 7A to 7D, i.e., with both positive and negative x-axiscomponents cx of the c director between a pair of substrates, theconditions as calculated below must be met. Specifically, equations (1),(6) and (7) indicate that the substrate interface layer tilt angle 5 is0° to 3° (0° is associated with the presence of a layer planeperpendicular from the substrate), and the c director pretilt 6 that thec director 4 providing a vector of projection of the liquid crystalmolecule on the layer plane forms with the component of the layer planeparallel with the substrate is 3° to 5°, or the substrate interfacelayer tilt angle 5 is 4° to 7° with the c director pretilt 6 of 0°(i.e., with the c director parallel with the substrate) to 3°, or thesubstrate interface layer tilt angle 5 is 8° to 20° with the c directorpretilt 6 of 9° to 90°.

Of these layer structures, the layer structures and the moleculararrangement as shown in FIGS. 3A to 3D have a threshold voltage lowerthan the layer structures and the molecular arrangement shown in FIGS.4A to 4D. Also, the layer structures and the molecular arrangement asshown in FIGS. 4A to 4D exhibit a threshold voltage lower than thoseshown in FIGS. 5A to 5D. Also, in view of the fact that each layerstructure is stable in terms of energy and the liquid crystal cellprepared according to the first embodiment has a plurality of layerstructures in one pixel, a liquid crystal device highly resistant tostress can be produced and selective partial drive is possible accordingto the difference of the threshold voltage, thus permitting gray scaledisplay.

Further, in order to confirm the long-term stability of the liquidcrystal cell prepared according to the second to fourth embodiments, thewhite write waveform and the transmitted light characteristics impressedwere measured. The white write waveform and the transmitted lightcharacteristics after voltage application are shown in FIGS. 9A and 9B,respectively, while the black write waveform and the transmitted lightcharacteristic after voltage application are shown in FIGS. 10A and 10B,respectively.

In order to evaluate the unilateral stability, the amount oftransmittance (TmW) one minute after the amount of transmittance (TW)immediately after application of the white write waveform shown in FIGS.9A, 9B and the amount of transmittance (TmB) one minute after the time(TB) immediately following the application of the black write waveformshown in FIGS. 10A, 10B were measured for each embodiment and thereference. The result is shown in Table 1 below.

                  TABLE 1                                                         ______________________________________                                                          White   Black                                                                 (TmW/TW)                                                                              (TmB/TB)                                            ______________________________________                                        2nd embodiment (FIGS. 3A to 3E)                                                                   1.0       1.0                                             3rd embodiment (FIGS. 4A to 4E)                                                                   1.0       1.0                                             4th embodiment (FIGS. 5A to 5E)                                                                   1.0       1.0                                             Reference (FIGS. 7A to 7E)                                                                        0.6       1.0                                             ______________________________________                                    

Table 1 shows that the liquid crystal cell having the layer structuresshown in FIGS. 7A to 7D of the reference has the amount of transmittance(TW) immediately after application of the white write waveform differentfrom the amount of transmittance (TmW) one minute later. On the otherhand, the liquid crystal cell having the layer structures shown in FIGS.3A to 3D, 4A to 4D and 5A to 5D representing the second to fourthembodiments, respectively, were confirmed to have no difference in thetransmittance amount. This indicates that there is no difference incontrast between the time of driving and the time of storage.

Also, FIG. 11 shows the measurement of secular variations between thetransmittance amount after application of the white write waveform andthe transmittance amount after application of the black write waveform.The transmittance amount on the ordinate indicates an amountstandardized using the transmittance amount (TW) immediately afterapplication of the white write waveform and the transmittance amount(TB) immediately after application of the black write waveform. Theabscissa represents the number of days that the display was allowed tostand. White circles (101) represent a change in transmittance amount atthe time of application of the white write waveform for the liquidcrystal cell according to the invention shown in FIGS. 3A to 3D. Whitetriangles (102), on the other hand, represent a change in transmittanceamount at the time of application of the white write waveform for theliquid crystal cell according to the reference shown in FIGS. 7A to 7D.Black circles (104) represent a change in transmittance amount at thetime of application of the black write waveform for the liquid crystalcell according to the invention shown in FIGS. 3A to 3D. Black triangles(103) indicate a change in transmittance amount at the time ofapplication of the black write waveform for the liquid crystal cell ofthe reference shown in FIGS. 7A to 7D. In the liquid crystal cellaccording to the invention shown in FIGS. 3A to 3D, the bistability wasstill exhibited 7 days after application (101) of the white writewaveform. With the liquid crystal cell (102) according to the reference,by contrast, it was confirmed that the black side gradually changes tothe unilaterally stable state higher in stability.

A unilateral stability evaluation test was similarly conducted on theliquid crystal cell having the configuration as shown in FIGS. 4A to 4Dand FIGS. 5A to 5D. Consequently, it could be confirmed that a long-termbistability can be secured as compared with the liquid crystal cell ofthe reference as in the case of the liquid crystal cell shown in FIGS.3A to 3D.

For the layer structures according to the invention to be formed, i.e.,for the substrate interface layer tilt angle and the c director pretiltto be formed within the angular range of the invention, various methodsare conceivable other than those of the embodiments of the inventiondescribed above. For example, a thin insulating film having coarseparticles is formed by sputtering on an ITO electrode, and anorientation film not mixed with chiral material is coated and rubbedthereon. After that, a ferroelectric liquid crystal is injected into theliquid crystal cell, followed by application of an AC electric field. Itwas confirmed that the resulting distribution of the particles of theinsulating film can form a plurality of layer structures including thoseshown above which are stable in terms of energy.

Further, the ferroelectric liquid crystal is injected into a liquidcrystal cell which is formed by depositing a thin metal film about 10 nmthick on an ITO electrode, coating an orientation film thereon andfurther rubbing the assembly. Then, the AC field processing is performedthereby to form a plurality of layer structures including layerstructures stable in terms of energy as described above. In the casewhere a plurality of layer structures are to be produced as describedabove by the above-mentioned AC field processing, the same effect can beachieved also by such means as making up structures with a distributedcell gap or producing a distribution of resistance value in theelectrode film.

The layer structures can also be controlled by utilizing the chiralityof the orientation film without the AC field processing. For example,the layer structures as described above can be produced by using aliquid crystal high polymer having a plurality of functional groups offerroelectric liquid crystal in the side chain.

Substantially the same can be said of the antiferroelectric liquidcrystal. In this case, however, it is easier and more positive tocontrol the layer structures in the ferroelectric phase than to controlthe layer structures in antiferroelectric phase or ferrielectric phase.A more accurate control operation is possible by the intermediary of theferroelectric phase in an electric field or a magnetic field at the timeof cooling from isotropic to room temperature.

In the second to fourth embodiments described above, the liquid crystalcells having the configurations shown in FIGS. 3A to 3D and FIGS. 5A to5D were prepared by changing the type of the orientation film materialfor one of the substrates. Even when the same orientation film materialis used, however, it was confirmed that the liquid crystal cells havingthe configurations shown in FIGS. 3A to 3D, FIGS. 4A to 4D and FIGS. 5Ato 5D can be obtained by changing the film-forming conditions. Theinventors have also confirmed that the conditions exist that can producea similar configuration with other combinations of the orientation filmmaterials.

FIELD OF INDUSTRIAL UTILIZATION

The configuration of a liquid crystal device having layer structures anda molecular arrangement with at least two types of substrate interfacelayer tilt angles shown in the present invention is not only superior inrigidity but also exhibits a threshold value characteristic differentdue to the special layer structures. Since different behaviors areexhibited according to the voltage applied at the time of driving, theswitching region in a pixel can be controlled for a ferroelectric liquidcrystal electro-optical device. In an antiferroelectric electro-opticaldevice, on the other hand, the region of field induced phase transitioncan be controlled. For this reason, an analog gray scale display havinga threshold voltage high in stress resistance is made possible.

Also, in the configuration of a liquid crystal device having layerstructures and a molecular arrangement with the c director pretiltarranged symmetrically on the two substrates according to the invention,the layer structures and the molecular arrangement at the time of whitewrite and black write are equivalent in terms of energy. Further, themolecular arrangement is stable with a substantially equal amount oftransmittance between the drive time and storage time. As a result, along-term bistability is assured without any difference in contrastbetween storage time and drive time. Consequently, a superior liquidcrystal electro-optical device can be very effectively obtained which ishigh in long-term reliability and free of flicker at the time ofdriving.

We claim:
 1. A liquid crystal device with liquid crystal representinglayer structures held between a pair of parallel substrates each havingan electrode and a plurality of pixels formed between said electrodes,characterized in thatsaid layer structures are arranged in such a mannerthat there are at least two substrate interface layer tilt angles thatthe normal to a selected one of the substrates forms with the layerplane of said layer structures within the same pixel, and at least oneof said layer structures in said same pixel has said substrate interfacelayer tilt angles of 0° to 3°, and the c director pretilt that the cdirector providing a unit vector of projection of said liquid crystalmolecule on said layer plane at said substrate interface forms with thecomponent of said layer plane parallel with said substrate is 3° to 5°.2. A liquid crystal device with liquid crystal representing layerstructures held between a pair of parallel substrates each having anelectrode and a plurality of pixels between said electrodes,characterized in thatsaid layer structures are arranged in such a mannerthat there are at least two substrate interface layer tilt angles thatthe normal to a selected one of the substrates forms with the layerplane of said layer structures within the same pixel, and at least oneof said layer structures in said same pixel has said substrate interfacelayer tilt angles of 4° to 7°, and the c director pretilt that the cdirector providing a unit vector of projection of said liquid crystalmolecule on said layer plane at said substrate interface forms with thecomponent of said layer plane parallel with said substrate is 0° to 3°.3. A liquid crystal device with liquid crystal representing layerstructures held between a pair of parallel substrates each having anelectrode and a plurality of pixels between said electrodes,characterized in thatsaid layer structures are arranged in such a mannerthat there are at least two substrate interface layer tilt angles thatthe normal to a selected one of the substrates forms with the layerplane of said layer structures within the same pixel, said substrateinterface layer tilt angles of said layer structures being variablebetween said electrodes, and at least one of said layer structures insaid same pixel has said substrate interface layer tilt angles of 8° to20°, and the c director pretilt that the c director providing a unitvector of projection of said liquid crystal molecule on said layer planeat said substrate interface forms with the component of said layer planeparallel with said substrate is 9° to 90°.
 4. A liquid crystal deviceaccording to any one of claims 1 to 3, wherein said layer structureswithin said one pixel are partly or wholly formed asymmetric about asymmetry plane equidistant from said two substrates.
 5. A liquid crystaldevice according to claim 4, wherein said liquid crystal isferroelectric.
 6. A liquid crystal device according to claim 4, whereinsaid liquid crystal is antiferroelectric.
 7. A liquid crystal deviceaccording to any one of claims 1 to 3, wherein said layer structureswithin said one pixel are partly or wholly formed symmetric about asymmetry plane equidistant from said two substrates.
 8. A liquid crystaldevice according to claim 7, wherein said liquid crystal isferroelectric.
 9. A liquid crystal device according to claim 7, whereinsaid liquid crystal is antiferroelectric.
 10. A liquid crystal devicewith liquid crystal representing layer structures held between a pair ofparallel substrates each having an electrode and a plurality of pixelsformed between said electrodes, characterized in thatthe c directorproviding a unit vector of projection of the liquid crystal molecule onthe layer plane of said layer structures is arranged symmetrically abouta symmetry plane equidistant from said substrates at the substrateinterface in the same pixel, and the substrate interface layer tiltangle providing an angle that the normal to selected one of thesubstrates forms with the layer plane is 0° to 3°, and that the cdirector pretilt providing an angle that said c director forms with thecomponent of said layer plane parallel with said substrate is 3° to 5°.11. A liquid crystal device with liquid crystal representing layerstructures held between a pair of parallel substrates each having anelectrode and a plurality of pixels formed between said electrodes,characterized in thatthe c director providing a unit vector ofprojection of the liquid crystal molecule on the layer plane of saidlayer structures is arranged symmetrically about a symmetry planeequidistant from said substrates at the substrate interface in the samepixel, and the substrate interface layer tilt angle providing an anglethat the normal to selected one of the substrates forms with the layerplane is 4° to 7°, and the c director pretilt providing an angle thatsaid c director forms with the component of said layer plane parallelwith said substrate is 0° to 3°.
 12. A liquid crystal device with liquidcrystal representing layer structures held between a pair of parallelsubstrates each having an electrode and a plurality of pixels formedbetween said electrodes, characterized in thatsubstrate interface layertilt angles of said layer structures are variable between saidelectrodes, the c director providing a unit vector of projection of theliquid crystal molecule on the layer plane of said layer structures isarranged symmetrically about a symmetry plane equidistant from saidsubstrates at the substrate interface in the same pixel, and thesubstrate interface layer tilt angle providing an angle that the normalto selected one of the substrates forms with the layer plane is 8° to20°, and the c director pretilt providing an angle that said c directorforms with the component of said layer plane parallel with saidsubstrate is 9° to 90°.
 13. A lipped crystal device according to any oneof claims 1 to 3 or 10 to 12, wherein said liquid crystal isferroelectric.
 14. A liquid crystal device according to any one ofclaims 1 to 3 or 10 to 12, wherein said liquid crystal isantiferroelectric.